The leading cause of perinatal mortality and morbidity throughout the world today is premature delivery. Each year, approximately 250,000 pregnancies terminate prematurely in the United States alone. The perinatal wastage and the attendant costs involved in treatment and rehabilitation constitute major public health problems. Since 60% of premature deliveries occur without known cause, prevention has been notably unsuccessful heretofore. Accordingly, the incidence remains high in many obstetrical populations. Recent success in the prevention of prematurity in a high risk population, by the use of a long-acting progestational agent, suggests that hormonal abnormalities constitute a major etiologic factor in premature labor. Selected hormonal studies in this high risk population may identify the specific abnormalities at fault, and thereby lead to improved diagnosis and treatment in the prevention of prematurity. The objectives of this project are: 1) To determine those maternal hormonal changes that precede the onset of premature labor; and 2) to determine the mechanism of action and dose-response relationship of 17 gamma-hydroxyprogesterone caproate in the prevention of prematurity. A previously identified high risk obstetrical population will be employed for this study. Sequential plasma endocrine analyses will be performed and compared in those patients receiving no hormonal therapy and in those receiving 17 gamma-hydroxyprogesterone caproate.